The invention relates to a method and an apparatus for damping the pressure peak occurring at the end of the die filling phase in die casting machines. The apparatus includes a hydraulic damping device disposed between the piston rod of the drive piston and the piston rod of the casting piston. The damping chamber of the damping device is bounded by a control piston through which no fluid flows when damping takes place and which is not required to move against the direction of movement of the casting piston rod in order to open a control port with its control edge.
The die casting process involves essentially three operating phases. During the first operating phase, the pressing plunger proper initiates the pressing process at a relatively slow speed, thus enabling the air still contained in the injection cylinder to escape. In the second operating phase, in which the pressing plunger moves further at an increased speed, the liquid metal is pressed into the die. In the subsequent third operating phase, the metal is compressed in the die by a so-called dwell pressure.
The present invention essentially relates to the second operating phase, at the end of which the pressing plunger and its piston rod are suddenly braked to a zero speed. This generates in the metal a pressure peak which may overcome the die closing force proper, and thus a brief opening of the die cannot be excluded.
To prevent such an undesired die opening, high closing forces are used which would actually not be necessary at the end of the second operating phase if the pressure peak were avoided.
A measurement of the pressure peak by means of appropriate devices shows that in reality there are two pressure peaks. The first pressure peak occurs immediately at the end of the rapid die filling process and is only due to the sudden braking of the moving masses including the casting piston rod, the coupling and the plunger rod. After about 1 to 2 milliseconds, a hydraulic pressure peak follows as a result of the moving oil column being braked. The intensity of the first pressure peak can be measured only in the die or by a comparable measuring device while the second pressure peak is measured as usual in the drive chamber of the pressing system.
If the casting operation is performed without, the casting piston is braked within 1 to 2 milliseconds, dependent from the volume to be cast and the state of the die-casting mold, from its charging speed to a standstill.
To make the transition as smooth as possible, the switching process must thus last less than one millisecond. The greater the switching delay, the greater the reduction in speed and thus the magnitude of the pressure peak.
A further factor contributing to the magnitude of the pressure peak is the pressure captured in the damping chamber. In a system in which the pressure in the damping chamber opens the control valve against a spring force, the biasing force of the spring must be higher, i .e. the maximum operating and acceleration pressure in the die filling phase. Accordingly, at the end of die filling, the control pressure is first increased and only if the spring force has been exceeded to an appropriate degree, is the control valve switched. This results in the loss of valuable time. Furthermore, the control pressure must be added to the pressure peak of the kinetic energy since the movement of the control piston begins only after this switching period.
German patent document 2,818,061 discloses a damping device of the abovedescribed configuration.
In this device too, a damping chamber is controlled by a spring-charged control piston which is likewise disposed in the direction of movement of the casting piston rod. This control is designed in such a manner that the control valve operates as a pressure valve and the mass of the control valve is intentionally maintained at a small value.
This prior art construction has the following drawbacks:
(a) the opening force of the control valve must be higher than the transfer force of the piston rod in the die filling phase;
(b) fluid passes through the control valve, i.e. the hydraulic medium present on the spring side of the control valve must be displaced; this influences the response time of the control valve;
(c) the reservoir chamber of the damping device is, due to its structure, smaller than the displaced volume so that an additional reservoir space must be provided.
It is also known (German Patent No. 2,833,063) to equip the casting piston with hydraulic damping means. In this case, the casting piston rod is displaceably mounted in a hollow casting piston. A piston antechamber then accommodates a hydraulic medium which serves as damping and cooling means.
Each piston configuration requires a correspondingly adapted damping device.
Since the piston is a part that is subject to wear, higher operating costs inevitably result upon replacement, since a piston equipped with a damping device is considerably more expensive than a simple casting piston. Sealing problems also result since the casting piston is subjected to very large differences in temperature.
During damping, the control piston responsible for the response time of the damping device must be accelerated against its direction of movement during the die filling process and against the spring force which is set to correspond to the maximum operating pressure in the damping chamber and must be moved over a relatively long path.
German patent document 3,433,121 discloses a device which corresponds to the embodiment now being presented, except that it also includes a spring-charged control piston. The drawback of this device is that the force exerted by the decelerated mass of the control piston is in part consumed by the pretensioned spring force and thus, for lower die filling speeds, the response times are correspondingly longer.